In May 2012, while hiking the Appalachian Trail through the park, I photographed several flies at Double Spring Shelter. One had apparently died face down in a buttercup. Others were clinging to unfurling beech leaves. All of them had swollen abdomens striped white and brown (or pinkish and gray) on the upper side and white (or pink) underneath and were pitched at a strange angle with wings and legs spread, abdomens raised. (June 2012 Archive, AT Day Four and AT Day Five)
When writing those blog posts, I could neither determine the species of fly nor discover an explanation for their behavior. In December 2013, 19 months after this unusual sighting, I found the answer to both mysteries quite by accident.
I have been researching the Kingdom Eumycota (fungi) for the Tennessee Naturalist Program. I serve on their board and teach their class on plants and fungi at Owl’s Hill Nature Sanctuary near Nashville. TNP is developing detailed curriculum study guides for each class topic, and I am writing the guide for “Forbs, Ferns, Fungi, and More.”
The fungi kingdom is still rather new (comparatively speaking — fungi used to be part of the plant kingdom) and is undergoing significant reorganization as more fungi are identified and classified, especially with DNA studies. This has wreaked havoc on the kingdom’s phyla (main classification ranks), and I was trying to piece together the current picture, particularly with Zygomycota. Fungi in this phylum are quite diverse and include parasitoids, organisms that live part or most of their lives within a host and usually kill that host.
A few well-known species of parasitoid fungi are in the genus Entomophthora, a Latin name that means “insect destroyer.” Sticky spores of these fungi float through the air, and when one reaches a fly, it adheres to the body, germinates, and produces a hypha that penetrates the fly (usually through a soft segment membrane), and begins to grow throughout the fly’s body. It grows so much, the dark abdominal segments spread apart exposing light colored membranes in between and imparting a striped appearance. Then comes the creepy part.
The fungus invades the fly’s brain and alters its behavior. The fly crawls up to a high place just before it dies, cements its proboscis to a surface, flares open its wings, stretches its hind legs, and raises its abdomen up and out. This bizarre posture of death optimizes the chances for spores to infect new hosts. When the fly dies, sporangia burst through the segments and abdominal wall, forcibly discharging spores up to 3/4 inch away, a decent distance for a microscopic spore. These spores may form a whitish halo on and around the fly. The timing of all this often occurs in the hours before midnight, when the humidity is higher and environmental conditions are most favorable for spore production.
One final fungus note: Should this infection and death occur late in the year, spores are not produced. The dead fly drops to the ground and so-called “resting spores” develop in the body to overwinter. In spring, the spores are discharged to begin a new year of infection cycles.
With the fungus identified, I begin researching the host flies. It only infects adults and has been found in several fly families, including common house flies, hover or flower flies, dung flies, flesh flies, blow flies, and tachinid flies, plus other invertebrates such as moths and aphids. There are several species of Entomophthora, each tending to infect certain insect species. These fungi are being examined as a biological control for pest species like house flies and gypsy moths.
After extensive review of the fly families frequently targeted by Entomophthora on BugGuide.net, I’m fairly certain the flies pictured here are in the family Scathophagidae or dung flies. The wing markings and leg hairs are a solid match.
Isn’t nature remarkable? …and sometimes scary? …and isn’t it a fantastic feeling to finally solve a year-and-a-half-old puzzle?
[Primary source: The Kingdom Fungi: the Biology of Mushrooms, Molds, and Lichens by Steven L. Stephenson, Timber Press, 2010.]